miRNAs regulate protein synthesis by suppressing translation or destabilizing mRNAs. Their role in the development and function of the nervous system is not well understood. In this year, we identified several miRNAs that regulates the development and function of synapses in the hippocampus. We found that expression of these miRNAs is regulated during the period of active synaptogenesis. We also searched for target mRNAs that are regulated by miRNAs. We have identified respective mRNA targets for these miRNAs and demonstrated that the expression level of target mRNAs inversely correlates with that of the miRNA. In addition, by using overexpression and knockdown approaches, we show that the regulation of miRNAs on synapse development is mediated by their target mRNAs. These findings indicate that miRNAs play important roles in the development of synapses.